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Sharks use whip-like tail to stun prey

Thresher sharks hunt schools of sardines by whipping their tails over their heads to stun several fish at the same time, say scientists.

It has long been thought that these sharks use their tails when hunting for food, but the mechanism had been poorly understood, until now.

The research, published today in PLoS ONE documents two types of tail-slap, over-the-head and sideways tail-slaps. But in 25 instances of tail-slapping, 22 were of the overhead kind.

To capture their prey with one of these overhead slaps, the sharks lunge towards a shoal of fish, then pull their pectoral fins - the ones near their chest - in towards each other, which acts as a braking mechanism. This sudden brake causes their tail fins to lift up rapidly over their heads and slap both the fish and the water.

Thresher shark.

During the study, the quickest slap, made by the largest shark with the longest tail, was measured at speeds of nearly 48 miles per hour.

The researchers observed bubbles in the water after this and similar fast slaps, something which is seen after a shockwave moves through the water. It has long been known that when killer whales use their tails to stun fishes, the whipping motion produces shockwaves. So, it's likely Thresher sharks do too, and may be the reason they are able to stun so many prey with one tail-slap.

'The really forceful slaps resulted in dissolved gases coming out of the water, so we can hypothesise that the behaviour causes shockwaves in the water column that are powerful enough to stun the sardines,' explains Dr Simon Oliver, who was a NERC PhD student when he led the research and founded The Thresher Shark Research and Conservation Project. 'I suspect the shockwaves are an important part of the hunting mechanism and that they help facilitate the animal's ability to stun and maim more than one prey at a time.'

The discovery was made using video footage captured by citizen-scientist divers, off of the coast of a small coral island in the Philippines, after Oliver was alerted to a group of the vulnerable species feeding on a shoal of sardines in the area.

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The three-and-a-half-metre long sharks regularly prey on large, dense shoals of fish, called bait balls, and it seems that whipping their tails in this way makes feeding more efficient.The shockwaves may mean the shark is able to stun many fish at a time and capture multiple prey with little effort.

'This is appears to be an efficient method for hunting schooling prey, rather than a shark chasing sardines in a school of hundreds of thousands,' says Oliver.

Other animals, like killer whales and dolphins also use this energy-efficient method of catching prey.

'This extraordinary story highlights the diversity of shark hunting strategies in an ocean where top predators are forced to adapt to the complex evasion behaviours of their ever declining prey,' he concludes.

The work was supported by a NERC studentship, awarded to Simon Oliver.